feat(tools): Add CallableRetrieval for custom retrieval backends

Add a new CallableRetrieval class that wraps any user-provided sync or async callable as
a BaseRetrievalTool, enabling custom retrieval backends (Elasticsearch, Pinecone, pgvector, etc.)
to have first-class retrieval semantics without requiring specific framework dependencies.

Author: savula15

src/google/adk/tools/retrieval/__init__.py

@@ -13,9 +13,11 @@
 # limitations under the License.
 
 from .base_retrieval_tool import BaseRetrievalTool
+from .callable_retrieval import CallableRetrieval
 
 __all__ = [
     "BaseRetrievalTool",
+    "CallableRetrieval",
     "FilesRetrieval",
     "LlamaIndexRetrieval",
     "VertexAiRagRetrieval",

src/google/adk/tools/retrieval/callable_retrieval.py

@@ -0,0 +1,78 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Retrieval tool that wraps a user-provided callable."""
+
+from __future__ import annotations
+
+import inspect
+from typing import Any
+from typing import Awaitable
+from typing import Callable
+from typing import Union
+
+from google.adk.tools.retrieval.base_retrieval_tool import BaseRetrievalTool
+from google.adk.tools.tool_context import ToolContext
+from typing_extensions import override
+
+
+class CallableRetrieval(BaseRetrievalTool):
+  """Retrieval tool backed by a user-provided function.
+
+  Wraps any callable that accepts a query string and returns results,
+  making it a first-class retrieval tool in ADK.
+
+  Example:
+      >>> def search_docs(query: str) -> list[str]:
+      ...     return my_db.search(query)
+      >>> tool = CallableRetrieval(
+      ...     name="search_docs",
+      ...     description="Search the knowledge base.",
+      ...     retriever=search_docs,
+      ... )
+
+  Args:
+      name: Tool name exposed to the LLM.
+      description: Tool description exposed to the LLM.
+      retriever: A sync or async callable. Must accept a ``query``
+          string as its first argument. May optionally accept a
+          ``tool_context`` parameter.
+  """
+
+  def __init__(
+      self,
+      *,
+      name: str,
+      description: str,
+      retriever: Union[
+          Callable[[str], Any],
+          Callable[[str], Awaitable[Any]],
+      ],
+  ):
+    super().__init__(name=name, description=description)
+    self._retriever = retriever
+    self._pass_tool_context = (
+        "tool_context" in inspect.signature(retriever).parameters
+    )
+
+  @override
+  async def run_async(
+      self, *, args: dict[str, Any], tool_context: ToolContext
+  ) -> Any:
+    query = args["query"]
+    kwargs = {"tool_context": tool_context} if self._pass_tool_context else {}
+    result = self._retriever(query, **kwargs)
+    if inspect.isawaitable(result):
+      return await result
+    return result

tests/unittests/tools/retrieval/test_callable_retrieval.py

@@ -0,0 +1,167 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from unittest.mock import MagicMock
+
+from google.adk.tools.retrieval.base_retrieval_tool import BaseRetrievalTool
+from google.adk.tools.retrieval.callable_retrieval import CallableRetrieval
+from google.adk.tools.tool_context import ToolContext
+import pytest
+
+
+@pytest.fixture
+def mock_tool_context():
+  return MagicMock(spec=ToolContext)
+
+
+def test_isinstance_base_retrieval_tool():
+  tool = CallableRetrieval(
+      name="test",
+      description="A test tool.",
+      retriever=lambda query: [],
+  )
+  assert isinstance(tool, BaseRetrievalTool)
+
+
+def test_get_declaration():
+  tool = CallableRetrieval(
+      name="my_search",
+      description="Search docs.",
+      retriever=lambda query: [],
+  )
+  declaration = tool._get_declaration()
+  assert declaration.name == "my_search"
+  assert declaration.description == "Search docs."
+
+
+@pytest.mark.asyncio
+async def test_sync_callable(mock_tool_context):
+  def my_retriever(query: str):
+    return [f"result for {query}"]
+
+  tool = CallableRetrieval(
+      name="sync_tool",
+      description="A sync retrieval tool.",
+      retriever=my_retriever,
+  )
+  result = await tool.run_async(
+      args={"query": "hello"}, tool_context=mock_tool_context
+  )
+  assert result == ["result for hello"]
+
+
+@pytest.mark.asyncio
+async def test_async_callable(mock_tool_context):
+  async def my_retriever(query: str):
+    return [f"async result for {query}"]
+
+  tool = CallableRetrieval(
+      name="async_tool",
+      description="An async retrieval tool.",
+      retriever=my_retriever,
+  )
+  result = await tool.run_async(
+      args={"query": "world"}, tool_context=mock_tool_context
+  )
+  assert result == ["async result for world"]
+
+
+@pytest.mark.asyncio
+async def test_tool_context_passthrough(mock_tool_context):
+  received_context = {}
+
+  def my_retriever(query: str, tool_context: ToolContext):
+    received_context["ctx"] = tool_context
+    return ["with context"]
+
+  tool = CallableRetrieval(
+      name="ctx_tool",
+      description="Tool with context.",
+      retriever=my_retriever,
+  )
+  result = await tool.run_async(
+      args={"query": "test"}, tool_context=mock_tool_context
+  )
+  assert result == ["with context"]
+  assert received_context["ctx"] is mock_tool_context
+
+
+@pytest.mark.asyncio
+async def test_tool_context_omission(mock_tool_context):
+  def my_retriever(query: str):
+    return ["no context needed"]
+
+  tool = CallableRetrieval(
+      name="no_ctx_tool",
+      description="Tool without context.",
+      retriever=my_retriever,
+  )
+  result = await tool.run_async(
+      args={"query": "test"}, tool_context=mock_tool_context
+  )
+  assert result == ["no context needed"]
+
+
+@pytest.mark.asyncio
+async def test_async_callable_with_tool_context(mock_tool_context):
+  async def my_retriever(query: str, tool_context: ToolContext):
+    return [f"async {query} with context"]
+
+  tool = CallableRetrieval(
+      name="async_ctx_tool",
+      description="Async tool with context.",
+      retriever=my_retriever,
+  )
+  result = await tool.run_async(
+      args={"query": "test"}, tool_context=mock_tool_context
+  )
+  assert result == ["async test with context"]
+
+
+@pytest.mark.asyncio
+async def test_sync_callable_object(mock_tool_context):
+
+  class MyRetriever:
+
+    def __call__(self, query: str):
+      return [f"object result for {query}"]
+
+  tool = CallableRetrieval(
+      name="obj_tool",
+      description="Callable object tool.",
+      retriever=MyRetriever(),
+  )
+  result = await tool.run_async(
+      args={"query": "hello"}, tool_context=mock_tool_context
+  )
+  assert result == ["object result for hello"]
+
+
+@pytest.mark.asyncio
+async def test_async_callable_object(mock_tool_context):
+
+  class MyAsyncRetriever:
+
+    async def __call__(self, query: str):
+      return [f"async object result for {query}"]
+
+  tool = CallableRetrieval(
+      name="async_obj_tool",
+      description="Async callable object tool.",
+      retriever=MyAsyncRetriever(),
+  )
+  result = await tool.run_async(
+      args={"query": "world"}, tool_context=mock_tool_context
+  )
+  assert result == ["async object result for world"]